The present invention relates to a laminated piezoelectric actuator and particularly, to a construction of a connecting portion of the piezoelectric actuator in which a plurality of electrostrictive effect elements are connected with each other in their longitudinal displacing direction.
Conventionally, as an actuator utilizing the electrostrictive effect, a laminated piezoelectric actuator which allows a large displacement to be achieved has been used. This actuator has been made in the following manner: After a film of piezoelectric ceramic material is formed with a thickness of about 100 .mu.m and an electroconductive paste is printed on its surface, it is cut into a predetermined size to form sheets, which are laminated by tens to hundreds of layers. On each surface of this laminated structure, a sheet of piezoelectric ceramic material without the electroconductive paste is laminated by several to tens of layers. It is sintered and polished in order to make its thickness constant. Thereafter, cutting and insulating operations and formation of leads are carried out to thereby obtain an electrostrictive effect element in which the distance between electrodes is small and the number of laminated layers is large.
The displacement of the electrostrictive effect element thus obtained accounts for about 10 .mu.m to 20 .mu.m (when 150 V is applied). If a further large displacement is required, then it is possible by increasing the number of laminated layers of the above-described sheet but, if the number of the laminations is increased, a crack or peeling can take place at the time of sintering.
Therefore, conventionally, in order to obtain a further large displacement, the electrostrictive effect elements obtained as above have been adhered to each other with an adhesive agent to form a stack, and it has been sealed with an enclosure consisting of a metallic casing and metallic members to make the actuator.
FIG. 1 is a longitudinal view in cross section illustrating the construction of such a conventional laminated piezoelectric actuator.
Referring to FIG. 1, a plurality of electrostrictive effect elements 1 are adhered to each other with an adhesive agent 11 to form a stack and the electrodes of these elements are connected to lead terminals 2a, 2b via leads 4a, 4b. Two disc-shaped metallic members 3, 6 are joined to aperture portions 5a, 5b of a cylindrical metallic casing 5. The upper and lower end portions 1a and 1b of the stack of electrostrictive effect element 1 are each adhered to the recess portions each formed on the inner surface of the metallic members 3 and 6 by means of the adhesive agent 11.
Incidentally, although, in general, the displacement of the electrostrictive effect element utilizes a longitudinal strain of the piezoelectric ceramic, which is caused by the applied voltage, at the same time, a transverse strain also takes place, which magnitude is proportional to the strength of the electric field. As a result, when the voltage is applied, the strength of the electric field of the piezoelectric ceramic at the adhered portion amounts to the order of one tenth to one thousandth of that of the piezoelectric ceramic at the other portion, and the transverse strain also assumes a similar difference. In order to absorb this difference in the transverse strains, as for the shape of the electrostrictive effect element, the piezoelectric ceramic at the adhered portion is deformed in a convex form. In consequence, when a plurality of elements are adhered, the adhered surface tends to deform into the convex form each time the voltage is applied to the piezoelectric actuator, causing a large stress in the neighborhood of the adhered portion. Therefore, if the voltage is repeatedly turned on and off, a peeling can take place at the adhered layer or a crack can take place on the piezoelectric ceramic in the neighborhood of the adhered layer.
In addition, depending on the material or thickness of the adhesive agent, the displacement can be absorbed and reduced by the order of tens of percent at each adhered layer.